Energy and particle transport rates in magnetically confined plasmas are often larger than neo-classical transport owing to binary collisions would allow. Anomalous transport, a major road block on the path to an economic fusion reactor, is a consequence of electric and magnetic fluctuations driven to supra thermal levels by various instability mechanisms. The linearly excited modes saturate by inducing a relaxation of the equilibrium profiles towards the marginally stable state, on the one hand, and via various non-linear interaction mechanisms, on the other hand. Specific instabilities, profile relaxation and non-linear interaction models are described and their successes and drawbacks are analysed in the light of observed characteristics of plasma confinement. A rough evaluation of the nuclear heating power required to balance the anomalous losses in the International Tokamak Experimental Reactor (ITER) is derived on the basis of the very qualitative mixing length estimate applied to electrostatic drift wave turbulence. Results from large-scale gyro-kinetic simulation codes are discussed.